Abstract
This thesis presents a detailed investigation into the dynamic behaviour of members in offshore structures subjected to ship collision. The study includes the interaction of existing service load on the platform (pre-loading) with impact loads. Using non-linear numerical models, both quasi-static and dynamic impacts have been examined. It has been found that pre-loading produces a significant change in the shape and extent of damage in an impacted member. It also dramatically changes the ultimate impact load that can be resisted and the magnitude of the energy that can be absorbed by the impacted member prior to its collapse. Axial pre-loading has been found to change quite dramatically the dynamic characteristics of cylindrical members with certain geometries while it has no significant effect on others. Interaction of pre-loading and collision forces can result in the propagation of instability either in the impacted tubular member or in adjacent members of the structural frame. An experimental test programme has also been undertaken to test tubular members under a combination of static axial pre-loading and lateral dynamic impacts. Useful information has been obtained from the experimental work. This information covers the time history of the event, the final deformations of the impacted tubes and recorded images of the damage development. The impact tests have also been simulated using a non-linear finite element program. The non-linear dynamic numerical models have been shown to be quite efficient in simulating first impact, sequential rebounds and propagation of dynamic instability in the tubes subjected to severe impact loads. Large-scale tubular frames have been examined under lateral impacts using numerical models to study the in-frame behaviour of the impacted members. A phenomenon called here ‘dynamic shakedown and stabilisation of response’ has been observed with both in-frame and isolated members subjected to lateral impacts. The combined analytical, numerical and experimental investigations carried out in the current study produce comprehensive data on the behaviour of impacted tubes. Results of this study provide guidance for more accurate assessment of the real impact situation. The main contribution of this study, compared with previous investigations reported in the literature, is that both pre-loading and dynamic effects have been included.